rear footwell aka the shallow end :-(

[Andymac]

As made fairly obvious in the title, after some bad rain today i now have a rather large puddle in my driver side rear footwell, i was parked on a slight slope, with the lowest point being the rear of the car which probably helped a bit but im pretty sure its come from the heater vent in the footwell so can anyone tell me where to start and everything i need to check? I know the plenum drain may need unblocking and the pollen filter resealing so can someone give me a link to a walkthru? Also is there anythin else i need to check? Damn these cars have a lot of common problems lol its a good job i love it D

Thanks guys,
Andy